Elastography is a known modality in which the hardness of body tissue is measured and the distribution of the measured hardnesses is then visualized. Elastography is used to diagnose diseases, such as liver cirrhosis, in which the hardness of body tissue changes according to the advancement of lesions. In elastography, there are two main methods to evaluate hardness, and in both methods the body tissue is displaced.
In the first method, the relative hardness of body tissue is visualized by using the magnitude of distortion at each point along a scanning cross-section that is observed when pressure is applied to the body tissue from the body surface with an ultrasound probe and the pressure is then released. In the second method, an acoustic radiation force or mechanical oscillations are applied from the body surface, shear waves then cause displacement of the body tissue, and the displacement is observed at each point along a scanning cross-section over time. This displacement is used to determine the propagation speed of the shear waves and the elasticity is then determined. In the former method, the local magnitude of distortion depends on the dynamic force due to an ultrasound probe being manually moved and an evaluation is made of whether an area of interest is hard or soft relative to the areas around the area of interest. On the other hand, in the latter method, the absolute elasticity of an area of interest can be determined.
In the latter method, a characteristic of shear waves is that they are reflected at the interfaces between tissues of different hardnesses. When displacement due to reflected shear waves is thus observed, the displacement can lead to inaccurate determination of the propagation speed of the shear wave and accordingly an artifact can occur in the hardness image displaying the hardness of body tissue. Consequently, various types of technologies have been proposed that will suppress these artifacts that are attributable to reflected shear waves.